---
type: tutorial
title: 'Back on dry land. Take your blog from day to night, no island required!'
description: |-
  Tutorial: Build your first Astro blog —
  Build a light/dark theme toggle using only JavaScript and CSS
i18nReady: true
---
import Box from '~/components/tutorial/Box.astro';
import Checklist from '~/components/Checklist.astro';
import MultipleChoice from '~/components/tutorial/MultipleChoice.astro';
import Option from '~/components/tutorial/Option.astro';
import PreCheck from '~/components/tutorial/PreCheck.astro';
import { Steps } from '@astrojs/starlight/components';

Now that you can build Astro islands for interactive elements, don't forget that you can go pretty far with just vanilla JavaScript and CSS!

Let's build a clickable icon to let your users toggle between light or dark mode using another `<script>` tag for interactivity... with no framework JavaScript sent to the browser.

<PreCheck>
  - Build an interactive theme toggle with only JavaScript and CSS
  - Send as little JavaScript to the browser as possible!
</PreCheck>

## Add and style a theme toggle icon

<Steps>
1. Create a new file at `src/components/ThemeIcon.astro` and paste the following code into it:

    ```astro title="src/components/ThemeIcon.astro"
    ---
    ---
    <button id="themeToggle">
      <svg width="30px" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24">
        <path class="sun" fill-rule="evenodd" d="M12 17.5a5.5 5.5 0 1 0 0-11 5.5 5.5 0 0 0 0 11zm0 1.5a7 7 0 1 0 0-14 7 7 0 0 0 0 14zm12-7a.8.8 0 0 1-.8.8h-2.4a.8.8 0 0 1 0-1.6h2.4a.8.8 0 0 1 .8.8zM4 12a.8.8 0 0 1-.8.8H.8a.8.8 0 0 1 0-1.6h2.5a.8.8 0 0 1 .8.8zm16.5-8.5a.8.8 0 0 1 0 1l-1.8 1.8a.8.8 0 0 1-1-1l1.7-1.8a.8.8 0 0 1 1 0zM6.3 17.7a.8.8 0 0 1 0 1l-1.7 1.8a.8.8 0 1 1-1-1l1.7-1.8a.8.8 0 0 1 1 0zM12 0a.8.8 0 0 1 .8.8v2.5a.8.8 0 0 1-1.6 0V.8A.8.8 0 0 1 12 0zm0 20a.8.8 0 0 1 .8.8v2.4a.8.8 0 0 1-1.6 0v-2.4a.8.8 0 0 1 .8-.8zM3.5 3.5a.8.8 0 0 1 1 0l1.8 1.8a.8.8 0 1 1-1 1L3.5 4.6a.8.8 0 0 1 0-1zm14.2 14.2a.8.8 0 0 1 1 0l1.8 1.7a.8.8 0 0 1-1 1l-1.8-1.7a.8.8 0 0 1 0-1z"/>
        <path class="moon" fill-rule="evenodd" d="M16.5 6A10.5 10.5 0 0 1 4.7 16.4 8.5 8.5 0 1 0 16.4 4.7l.1 1.3zm-1.7-2a9 9 0 0 1 .2 2 9 9 0 0 1-11 8.8 9.4 9.4 0 0 1-.8-.3c-.4 0-.8.3-.7.7a10 10 0 0 0 .3.8 10 10 0 0 0 9.2 6 10 10 0 0 0 4-19.2 9.7 9.7 0 0 0-.9-.3c-.3-.1-.7.3-.6.7a9 9 0 0 1 .3.8z"/>
      </svg>
    </button>

    <style>
      #themeToggle {
        border: 0;
        background: none;
      }
      .sun { fill: black; }
      .moon { fill: transparent; }

      :global(.dark) .sun { fill: transparent; }
      :global(.dark) .moon { fill: white; }
    </style>
    ```

2. Add the icon to `Header.astro` so that it will be displayed on all pages. Don't forget to import the component.

    ```astro title="src/components/Header.astro" ins={4,9}
    ---
    import Hamburger from './Hamburger.astro';
    import Navigation from './Navigation.astro';
    import ThemeIcon from './ThemeIcon.astro';
    ---
    <header>
      <nav>
        <Hamburger />
        <ThemeIcon />
        <Navigation />
      </nav>
    </header>
    ```

3. Visit your browser preview at `http://localhost:4321` to see the icon now on all your pages. You can try clicking it, but you have not written a script to make it interactive yet.
</Steps>

## Add CSS styling for a dark theme

Choose some alternate colors to use in dark mode.

<Steps>
1. In `global.css`, define some dark styles. You can choose your own, or copy and paste:

    ```css title="src/styles/global.css"
    html.dark {
      background-color: #0d0950;
      color: #fff;
    }

    .dark .nav-links a {
      color: #fff;
    }
    ```
</Steps>

## Add client-side interactivity

To add interactivity to an Astro component, you can use a `<script>` tag. This script can check and set the current theme from `localStorage` and toggle the theme when the icon is clicked. 
<Steps>
1. Add the following `<script>` tag in `src/components/ThemeIcon.astro` after your `<style>` tag:

    ```astro title="src/components/ThemeIcon.astro" ins={9-37}
    <style>
      .sun { fill: black; }
      .moon { fill: transparent; }

      :global(.dark) .sun { fill: transparent; }
      :global(.dark) .moon { fill: white; }
    </style>

    <script is:inline>
      const theme = (() => {
        if (typeof localStorage !== 'undefined' && localStorage.getItem('theme')) {
          return localStorage.getItem('theme');
        }
        if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
          return 'dark';
        }
          return 'light';
      })();
          
      if (theme === 'light') {
        document.documentElement.classList.remove('dark');
      } else {
        document.documentElement.classList.add('dark');
      }

      window.localStorage.setItem('theme', theme);

      const handleToggleClick = () => {
        const element = document.documentElement;
        element.classList.toggle("dark");
        
        const isDark = element.classList.contains("dark");
        localStorage.setItem("theme", isDark ? "dark" : "light");
      }

      document.getElementById("themeToggle").addEventListener("click", handleToggleClick);
    </script>
    ```

2. Check your browser preview at `http://localhost:4321` and click the theme icon. Verify that you can change between light and dark modes.
</Steps>

<Box icon="question-mark">

### Test your knowledge

Choose whether each of the following statements describes Astro `<script>` tags, UI framework components, or both.

1. They allow you to include interactive UI elements on your website.

    <MultipleChoice>
      <Option>
        Astro `<script>` tags
      </Option>
      <Option>
        UI framework components
      </Option>
      <Option isCorrect>
        both
      </Option>
    </MultipleChoice>

2. They will create static elements on your site unless you include a `client:` to send their JavaScript to the client and run in the browser.

    <MultipleChoice>
      <Option>
        Astro `<script>` tags
      </Option>
      <Option isCorrect>
        UI framework components
      </Option>
      <Option>
        both
      </Option>
    </MultipleChoice>

3. They allow you to "try out" a new framework without requiring you to start an entirely new project using that tech stack.

    <MultipleChoice>
      <Option>
        Astro `<script>` tags
      </Option>
      <Option isCorrect>
        UI framework components
      </Option>
      <Option>
        both
      </Option>
    </MultipleChoice>

4. They allow you to reuse code you have written in other frameworks and you can often just drop them right into your site.

    <MultipleChoice>
      <Option>
        Astro `<script>` tags
      </Option>
      <Option isCorrect>
        UI framework components
      </Option>
      <Option>
        both
      </Option>
    </MultipleChoice>

5. They allow you to add interactivity without needing to know or learn any other JavaScript frameworks.

    <MultipleChoice>
      <Option isCorrect>
        Astro `<script>` tags
      </Option>
      <Option>
        UI framework components
      </Option>
      <Option>
        both
      </Option>
    </MultipleChoice>
</Box>

<Box icon="check-list">

## Checklist

<Checklist>
- [ ] I can use JavaScript for interactivity when I don't want to add a framework.
</Checklist>
</Box>

### Resources

- [Client-side `<script>` in Astro](/en/guides/client-side-scripts/)
